Power transmission

ABSTRACT

A system and method for pulse-width modulation of multiple solenoid-operated valves in an electrohydraulic servo system employing a microprocessor-based digital controller. Solenoid-energization pulse widths are identified by comparison of operator command input signals to feedback signals responsive to actual motion at the actuators and loads. During each modulation period, all solenoids are initially selectively simultaneously energized as a function of binary information stored in an energization control register, and the shortest energization pulse duration is identified. After a delay corresponding to such shortest duration, during which the microprocessor may perform other control functions, the solenoids associated with such shortest duration are selectively de-energized as a function of binary information stored in a de-energization control register. The next-shortest duration is then identified, and the process again delays for a time equal to the difference between such next-shortest duration and the previous duration. The process of identifying pulse durations of increasing length and correspondingly deenergizing the solenoids continues until all pulse durations have been accommodated or until the modulation period terminates, at which point the entire control process rebegins.

The present invention is directed to valve control systems, and moreparticularly to a system and method for pulse-width modulation ofmultiple solenoid valves employing a single timer.

BACKGROUND AND OBJECTS OF THE INVENTION

Fluid valve systems, such as electrohydraulic valve systems, forvehicles and other like applications often embody multiplesolenoid-operated hydraulic valves which require separate independentcontrol to obtain motion at the load desired by the operator. Oneexample of such a system would be control of the bucket arm of anexcavator, involving provision of a solenoid valve and actuator for eachof the horizontal and vertical degrees of freedom. It is conventionalpractice, even where control is implemented by a microsprocessor-basedcontrol package, to employ separate timers to pulse width modulate eachsolenoid valve, wasting computing power that could be used for otherpurposes.

It is a general object of the present invention to provide a fluid valvecontrol system and method for pulse-width modulation of multiplesolenoid valves employing a single timer, and specifically a singlemicroprocessor-based timer subroutine.

SUMMARY OF THE INVENTION

In implementation of the present invention, pulsewidth modulation ofeach of several solenoid valves necessary to obtain motion desired bythe operator is first determined in the usual manner. Preferably,operator input command signals indicative of desired motions arecompared with feedback signals from each actuator and load to obtaincorresponding error signals, which are then translated into apulse-width ratio of a total modulation pulse period. Each pulse-widthsignal is stored in a corresponding register, and is updatedperiodically as a function of a change in operator demand and/or motionat the actuator and load. The pulse-width signals are employed topulse-width modulate a periodic signal to each actuator solenoid toobtain desired motion.

In accordance with the present invention, a pulse-width modulationperiod common to all of the solenoid actuators is established andrepeated continuously. Separate control registers are likewiseestablished to control energization and de-energization of the varioussolenoids, with separate bits of each register indicating whether theassociated solenoid is to be energized or de-energized. At the onset ofeach modulation period, all solenoids are selectively energized as afunction of binary information in the energization control register. Thevarious pulse-width registers are polled in sequence to identify theshortest pulse duration, and control operation is delayed for a timecorresponding to this duration, during which the microprocessor mayperform other computation functions.

At the end of the shortest pulse duration time, the correspondingsolenoid is selectively de-energized as a function of associated binaryinformation in the deenergization control register. The next-shortestpulse duration is then identified by polling the pulse-width registers,and a difference is obtained between such next-shortest pulse durationand the previous pulse duration. Again, further control operation isdelayed for the time of this pulse duration difference, during which themicroprocessor may perform other control functions. At the end of thedifference duration time, the solenoid valve associated with suchnext-shortest pulse duration is selectively de-energized, again as afunction of binary information in the de-energization control register.The next-shortest pulse duration is then identified and the entireprocess repeated until all pulse durations in the pulse width registershave been implemented, or until the modulation period terminates, atwhich point the entire control process rebegins.

Thus, the control microprocessor is freed for other control functionsduring each of the delay intervals. The entire pulse-width modulationprocess for all solenoids may be readily performed in a singlesubroutine which essentially identifies and implements pulse-widthcontrol of each solenoid in order of increasing pulse duration duringeach modulation period. Use of separate energization and deenergizationcontrol registers storing binary information associated with the variousvalve solenoids additionally facilitates computation speed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objects, features and advantagesthereof, will be best understood from the following description, theappended claims and the accompanying drawings in which:

FIG. 1 is a schematic diagram of an electrohydraulic control system forimplementing the present invention; and

FIG. 2 is a flow chart illustrating operation of the solenoidpulse-width modulation subroutine in accordance with a presentlypreferred embodiment of the invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a microprocessor-based digital controller 10 isconnected by an a/d converter 12 to receive a pair of input commandsignals J1,J2 from an operator joystick 14. Signals J1,J2 may indicate,for example, motion desired by the operator at a load in twocontrollable directions or degrees of freedom. Controller 10 likewisereceives and periodically samples a signal TPER indicating desiredmodulation period from an operator variable resistor 16 of othersuitable input means. (For a given application, TPER may be constant andfixed by programming without external input.) Controller 10 providesdigital on/off outputs through the amplifiers 18,20 to the solenoids22,24 of a valve 26 which selectively feeds hydraulic fluid underpressure from a source 28 and a pump 30 to drive a cylinder 32. Thepiston of cylinder 32 is coupled to the load (not shown). A positionsensor such as a variable resistor 34 provides a signal indicative ofactual motion at the actuator and load to an input of a/d converter 12for sampling and comparison with the corresponding command signal fromjoystick 14.

Controller 10 likewise provides digital or on/off signals throughamplifiers 36,38 to the solenoids 40,42 of a valve 44 for selectivelyconnecting source 28 and pump 30 to the cylinder 46 for actuating asecond load (not shown). A sensor 48 feeds a corresponding signalindicative of motion at the second actuator and load to an input of a/dconverter 12. In general, controller 10 samples and compares each sensorsignal X1,X2 to the corresponding sampled command signal J1,J2, anddetermines the necessary pulse-width modulation of energization signalsto valve solenoids 22,24 and 40,42 to obtain motion at the actuators andloads desired by the operator. Techniques for deriving this informationare well-known in the art and need not be described further in thisapplication.

In accordance with the present invention, a number of registers areestablished within controller 10, and controller 10 is suitablyprogrammed to utilize the information contained therein in order toobtain necessary pulse-width modulation from a single timer and controlroutine. The registers are illustrated schematically in FIG. 1 andinclude, in the exemplary embodiment, a register PW1 for storinginformation indicative of pulse width to solenoid valve 26, a registerPW2 for storing pulse width to valve 44, an ONCON register for storingbinary information indicative of whether the various valve solenoids areto be energized in each modulation period, and an OFFCON registercontaining binary information indicative of whether the varioussolenoids are to be de-energized during a modulation period. Informationstored in the PW1, PW2 ONCON and OFFCON registers is continuously andperiodically updated in the usual manner as a function of desired andactual motion at the various actuators and loads. A pair of registersTNEW and TCUM are likewise established within controller 10 for storingvariable information employed during the control subroutine.

FIG. 2 is a flow chart illustrating a presently preferred embodiment ofsuch control subroutine. For purposes of explaining operation, it willbe assumed that the ONCON register is a four-bit register containing thedigital byte or word 0111, and that the OFFCON register contains thedigital word 0010. A one-bit in the ONCON register indicates that acorresponding solenoid is to be energized during a modulation period.Thus, assigning the bits of the ONCON register to solenoids 22,24,40,42in increasing order of significance, 0111 in the ONCON registerindicates that solenoids 22,24 and 40, but not solenoid 42, are to beenergized during each modulation period. Likewise, a one-bit in theOFFCON register indicates that the corresponding solenoid is to remainenergized--i.e. not to be de-energized--during each modulation period.Thus, a 0010 in the OFFCON register indicates that solenoids 22,40 areto be de-energized during each modulation period, but that solenoids 24is not to be de-energized. In the example under discussion, solenoid 42is not energized and remains off during the modulation periods. It willalso be assumed that PW1 is less than PW2, and that both PW1 and PW2 aregreater than zero and equal to or less than TPER.

Referring to FIG. 2, initially, the variable TCUM is first tested bycomparison with the preselected modulation period TPER, which may be onthe order of ten milliseconds, for example. At the initiation of amodulation period, this comparison is true, and all solenoids 22,24,40and 42 are energized as defined by corresponding bits of the ONCONregister. Thus, in the example under discussion, solenoids 22,24 and 40are energized by application thereto of corresponding signals throughamplifiers 18,20 and 36, while solenoid 42 remains de-energized. Thevariable TCUM is set equal to zero, and the variable TNEW is set equalto TPER. PW1 is then compared with TCUM, which has been set to zero, sothat the comparison is false. PW1 is then compared with TNEW, which hasbeen set equal to TPER, so that the comparison is true and TNEW is resetequal to PW1. PW2 is then compared with TCUM, which is still equal tozero, so that the comparison is false. PW2 is compared with TNEW, whichhas been set equal to PW1. Since it has been assumed that PW1 is lessthan PW2, such comparison of PW2 with TNEW will be false. A delay timeDT is then established as equal to the difference between TNEW and TCUM.Since TNEW is equal to PW1 and TCUM is equal to zero, DT will be setequal to PW1. Thus, during the first pass of the routine illustrated inFIG. 2, the various solenoids have been selectively activated as definedby the ONCON register, the shortest pulse width PW1 has been identified,and the variable delay time DT has been set equal thereto. TCUM is thenset equal to TNEW, and the process is delayed for time DT, equal at thistime to PW1, during which the microprocessor may perform other controlfunctions.

After a delay of time DT, the process recycles and TCUM, now equal toTNEW and PW1, is compared with TPER. This comparison is false and TNEWis again set equal to TPER. On this pass, when PW1 is compared withTCUM, such comparison is true, so that operation branches to de-energizesolenoids 22,24 as defined by bits zero and one of the OFFCON register.In the example under discussion, solenoid 22 is thus turned off, whilesolenoid 24 remains energized. PW2 is then compared with TCUM, which isstill equal to PW1, so that the comparison is false. PW2 is thencompared with TNEW, which is now equal to TPER, so that the comparisonprovides true. TNEW is then set equal to PW2 and a second delay time DTis identified as being equal to the difference between TNEW, now equalto PW2, and TCUM, still equal to PW1. Thus, upon second pass through thecontrol cycle, delay time DT is identified as the difference between theshortest pulse duration PW1 and the next-shortest pulse duration PW2.The variable TCUM is set equal to TNEW, and thus equal to PW2, and thecontrol process delays for the time DT. On the third pass through thecontrol cycle, TCUM is equal to PW2, so that the comparison of PW1 withTCUM is true, as is the comparison of PW2 with TCUM. Thus, solenoids40,42 are deenergized as defined by OFFCON bits two and three and, forthe four-solenoid example, the process is complete.

It will be appreciated that the principles of the invention, includingparticularly the process of FIG. 2, may be increased by addingadditional PW/TCUM comparison, and corresponding solenoidde-energization functions, into the flow chart immediately above thestep at which delay time DT is established. Likewise, in an eight-bitmicroprocessor, each of the ONCON and OFFCON registers may accommodatebinary information relating to eight solenoids. In a sixteen-bitmicroprocessor, information relating to sixteen solenoids may be storedin the ONCON and OFFCON registers. Although the invention has beendisclosed in detail with specific reference to electro-hydraulic valves,the principles thereof may be applied equally as well to control ofother fluid valve systems, such as electropneumatic valves.

The invention claimed is:
 1. A fluid servo system comprising first andsecond solenoid valve actuators for variably actuating respective firstand second loads, first and second sensing means respectively coupled tosaid actuators and loads to provide corresponding first and secondsensor signals respectively indicative of actuation at said actuatorsand loads, means for receiving first and second input command signals,and digital control means for providing first and second pulse-widthmodulated signals to said first and second solenoid valve actuators asrespective functions of differences between said first and second sensorsignals and the corresponding said first and second command signals,said digital control means comprisingmeans for determining acontinuously repeating modulation time period TPER common to both ofsaid actuators, first and second register means for respectively storingmodulation time durations PW1 and PW2 equal to or less than saidmodulation period TPER for said first and second actuators respectively,means for selectively energizing said first and second solenoidactuators at the onset of each said time period TPER, means fordetermining the lesser of said time durations PW1 and PW2, means forselectively de-energizing one of said solenoid actuators associated withthe lesser of said time durations PW1 and PW2 following termination ofsaid lesser duration, means for determining a difference between saidtime durations PW1 and PW2, and means for selectively de-energizing theother of said solenoid actuators following termination of saiddifference between said durations.
 2. The system set forth in claim 1wherein said means for selectively energizing and de-energizing saidsolenoids comprise third and fourth registers respectively storinginformation indicative of energization and de-energization modes at saidsolenoid actuators, energization and de-energization commands at eachsolenoid of said actuators being represented by corresponding bits insaid registers.
 3. In an electrohydraulic servo system which includes aplurality of hydraulic actuators coupled to associated valves, saidvalves being operated by a plurality of electrical solenoid meeans, andmeans for directing pulse-width modulated control signals to each ofsaid solenoid means for controlling operation of the associated saidvalves and actuators, a method of pulse-width modulating all of saidsolenoid means employing a single timer comprising the steps of:(a)establishing a repeating pulse-width modulation period common to all ofsaid solenoid means, (b) separately storing energization pulse durationswithin said period for each of said solenoid means, (c) selectivelyenergizing all of said solenoid means at the onset of each said period,(d) identifying the shortest said pulse duration and delaying for a timeequal to said shortest duration, (e) selectively de-energizing the saidsolenoid means associated with said shortest pulse duration, (f)identifying a difference between said shortest pulse duration and thenext-shortest said pulse duration, and delaying for a time equal to saiddifference, (g) selectively de-energizing the said solenoid meansassociated with said next-shortest pulse duration, and (h) repeatingsaid steps (f) and (g) using successively increasing pulse durationsuntil said period is terminated.